Unlike traditional manufacturing that relies on subtractive techniques (e.g., cutting, drilling) to remove material from bulk shapes to arrive at a final shape, additive manufacturing builds shapes through precisely adding and consolidating layers of material according to a three-dimensional (3D) digital model. Depending on the underlying material used, various consolidation techniques can be used to fuse layers to form the desired structure, such as powder bed fusion, direct energy deposition (welding, electron-beam and laser processing), photo-polymerization, material jetting, binder jetting, and extrusion.
Additive manufacturing is often used to create functional prototypes or components out of polymeric materials. Plastics, rubbers, and other polymeric materials are typically used since consolidation techniques for blending layers of these materials together are economical and readily available. Conversely, metallurgical challenges and reduced properties associated with consolidating layers of metallic materials (e.g., metals, alloys, compounds) often reduce or even prevent their use for additive manufacturing. As a result, additive manufacturing exhibits limited viability for metallic materials and is confined to highly specialized components for medical, aviation, or other unique purposes.
One of the main reasons that consolidation techniques are difficult for a metallic material is the underlying microstructure of the material. In particular, metals and alloys consist of a large number of irregularly shaped crystals, also referred to herein as grains. Although indiscernible to the naked eye, the sizes and arrangement of these grains dictate the material's properties, including its strength, ductility, fatigue durability, strain rate, and resistance to creep deformation, among other properties. As such, the sizes and arrangement of the grains typically depend on the thermal and deformation history of the metallic material.
When fabricating a metallic object by additive manufacturing, the material is usually heated and deposited in layers to form the desired structure. For instance, depositing a new layer of heated metallic material upon a substrate or prior layer enables some grains of the material to experience elongated growth as a result of epitaxial growth. The elongated grains typically grow together in one well-defined orientation (e.g., columnar orientations) with respect to the substrate or prior layer, which can diminish the growth rate of other grains positioned in the traverse direction. As a result, the microstructure of the deposited layer is dominated by a few elongated grains with similar crystal orientations causing the layer to have anisotropic properties (i.e., physical and mechanical properties that vary depending on direction of measurement). Since the anisotropic properties compound with the addition of more layers growing in similar ways, the additively manufactured structure can vary in tensile strength, ductility, and other properties, relative to conventional wrought or cast options that can undesirably impact its viability. These deficiencies can prevent metallic materials from being used to additively manufacture many types of structures, including structures with minimal design margins.
One technique currently used to reduce anisotropic properties in deposited layers involves applying a rolling wheel to impart surface deformation upon each layer prior to adding the subsequent layer. Although the rolling wheel technique can promote grain recrystallization within deposited layers, the size and space required to use the rolling wheel limits overall applicability to additively manufacturing processes depositing material with thicker features and simple geometrical shapes. Therefore, there is a need for a technology that can refine the microstructure of a deposited layer of metallic material during additive manufacturing that can accommodate structures with generic scale and types of designs.